Oil Separators

As per United States Department of Transportation, Federal Highway Administration (FHWA), Urban Drainage Design Manual, Hydraulic Engineering Circular (HEC) No. 22, 3rd edition, 2009

Urban Water Quality Practices

The purpose of an urban Best Management Practice (BMP) is to mitigate the adverse

impacts of development activity. BMPs can be employed for stormwater control benefits

and/or pollutant removal capabilities. Several BMP options are available and should be

carefully considered based on site-specific conditions and the overall management

objectives of the watershed. Following are the various kinds of BMPs that have been

historically used to provide water quality benefits.

Extended Detention Dry Ponds

Wet or Retention Ponds

Infiltration Trench

Infiltration Basin

Sand Filters

Water Quality Inlets

Vegetative practices

Among the all, Water Quality Inlets are suitable in storm drainage systems for removing

sediment, oil and grease.

Water Quality Inlets

Water quality inlets are pre-cast storm drain inlets that remove sediment, oil, grease and

large particulates from parking lot runoff before it reaches storm drainage systems.

They are commonly known as oil and grit separators. Water quality inlets typically serve

highway storm drainage facilities adjacent to commercial sites where large amounts of

vehicle wastes are generated, such as gas stations, vehicle repair facilities, and loading

areas. They may be used to pretreat runoff before it enters an underground filter

system. The inlet is a three-stage underground retention system designed to settle out

grit and absorbed hydrocarbons.

An oil and grit separator consists of three chambers as shown in Figure 1, a sediment

trapping chamber, an oil separation chamber, and the final chamber attached to the

outlet. The sediment trapping chamber is a permanent pool that settles out grit and

sediment, and traps floating debris. An orifice protected by a trash rack, connects this

chamber to the oil separation chamber. This chamber also maintains a permanent pool

of water. An inverted elbow connects the separation chamber to the third chamber.

Figure 1: Cross-section detail of a typical oil/grit separator

Advantages of the water quality inlets lie in their compatibility with the storm drain

network, easy access and capability to pretreat runoff before it enters storm drainage

systems.

Disadvantages include their limited stormwater and pollutant removal capabilities, the

need for frequent cleaning (which cannot always be assured), the possible difficulties in

disposing of accumulated sediments and costs.

As per American Association of State Highway and Transportation Officials (AA SHTO), Highway Drainage Guidelines, 4th edition, 2007

Water Quality Inlets:

These structures (Figure 2), also known as oil and grit separators, consist of a series of

chambers designed to trap and retain sediments and hold floatables (e.g., oil, debris).

Sizing of the storage chambers is generally based on contributing drainage area and,

therefore, cost and size limit their application to relatively small discharges. These

facilities require high levels of maintenance to remove accumulated pollutants. Use of

water quality inlets is best limited to controlled runoff applications (e.g., maintenance

yards, parking lots and certain industrialized areas) where high concentrations of oils

are expected.

Fig 2. Water Quality Inlets

Based on the comparison between AASHTO and FHWA Water Quality Inlets are

proposed in areas wherever applicable (e.g., Parking area).The other aspects including

Location, Type and Quantity are to be finalized during Detail Design stage.